The present invention relates to testing of pulmonary functions and more specifically to the measurement of the forced expiratory volume for one second (FEV.sub.1) and the forced vital capacity (FVC).
Determining and detecting lung function failure, such as various types of emphysema, is presently an established practice to use various types of spirometric examinations, in which the vital capacity and forced expiratory volume at predetermined intervals are measured for comparison with given values. The presence of a failure in the lung function is detected through comparison of the subject's vital capacity with theoretical values which are dictated by the sex, age and height of the subject. The extent and type of the failure can be determined by correlation between the vital capacity and the time forced expiratory volume. When determining respiratory function, a medical practitioner may use a number of criteria for determining the condition of the patient. Two often used criteria are the forced vital capacity (FVC) of the patient's lungs and their forced expiratory volume timed for one second (FEV.sub.1). The ratio of these two volumes (FEV.sub.1 /FVC) is also used for diagnostic purposes. In normal patients the ratio of FEV.sub.1 /FVC is greater than 75% (0.75). A ratio of less than 75% is indicative of an obstructive impairment, such as asthma or emphysema.
Typical prior art spirometers use an expansible chamber in which the patient exhales. A scale measures the expansion of the chamber to determine the volume exhaled. Chambers can be a bellows-type or a bag having graduations marked thereon, which after exhaling into, can have the air pushed to one end to determine the volume exhaled into the bag. Another technique used in spirometers include hot wires in which resistance is varied in accordance with the cooling of the wire by air being blown through a tube. More commonly, a flow meter is used having rotating vanes rotated by a patient's breath being blown through the vanes thereby rotating the vanes. In order to measure the FEV.sub.1, a determination of the volume of air exhaled in the first second must be known and therefor must be timed in some manner. A commonly used spirometer uses an expansible chamber into which the patient exhales driving a pen over a clock driven chart calibrated so that a one second period can be determined. Such an instrument is bulky and expensive and requires readings to be taken from the chart. Accordingly, physicians are apt not to test pulmonary functions during routine examination. It has also been suggested to cut off the breathing tube in a spirometer after one second, but this sudden cut off might have adverse consequences for some patients.
One advantage of the present invention is to provide a spirometer of high accuracy which is small in size and does not require large expansible chambers to collect the air exhaled from the lungs and which can be used routinely in physical examinations and does not have a sudden blockage of the air being exhaled. It is also an advantage of the present invention to provide embodiments which can be manufactured at a low cost so that patients can obtain spirometers while in the hospital, or even for home use.
The present invention uses a pressure signal analgous to the volume passing through a tube rather than collecting the total volume of air in an expansible chamber, thus eliminating the much larger expansible chamber or clumsy bags, while providing an accurate signal representative of FEV.sub.1 and FVC and even of a ratio of FEV.sub.1 over the FVC desired. It is intended that the present invention will be used with a simplified chart or nomograph based on sex, age, height of the subject to screen patients to determine who should seek medical help or further testing in a pulmonary testing center.
An aspirator is generally defined as a suction pump operated by the pressure differential created by the high speed flow of a fluid past an intake orifice.
A venturi uses a narrow section in a passageway to increase the velocity of a fluid passing thereby to create a pressure differential between the narrowed section and either side thereof. A venturi is used to measure the flow of fluid.
Flow is the volume of fluid that flows through any given section of a passageway during a unit of time, thus the integral or totaled suction or negative pressure generated by a volume of fluid passing through a passageway across an aspirator orifice is analogous to the total volume passing that point and if a reading is taken at timed intervals, an indication of volume passing through the passageway during a predetermined period of time may be measured.